Highest i would run in a 45 is a mag 5 maybe a mag 7. My 180 gallon is on a Mag 24 only because of the head height is 6' and i have two returns so it t's off right at the sump. I use a Mag 7 on my protein skimmer and a mag 5 on my uv
sterilizer. then like AquaRick said add some power heads, im running the sicce voyager 3600gph and 4000gph kit with the wave controller. go up to a mag 7 on return and your more than doubling the output on your return. But you may end up blowing sand all over your tank.